The present invention relates to an electron beam-exposing method. More particularly, it relates to a method for exposing workpieces to an electron beam which is capable of describing fine patterns with high precision.
In a vector-scanning technique, an electron beam addresses only the pattern areas requiring exposure. A usual approach is to decompose the pattern into a series of simple shapes, such as rectangles and parallelograms. Vector scanning is usually more efficient than raster scanning but requires a higher performance-deflection system. In addition, it has several other advantages which raster scanning does not. For instance, ease of correction of the proximity effects of electron scattering and a significant compacting of data that leads to a much simpler control system.
Proximity effects are created by scattered electrons in the resist and by reflected electrons from the substrate. The electrons partially expose the resist up to several micrometers from the point of impact. As a result, a serious variation of exposure occurs over the pattern area when pattern geometries fall within the micrometer and submicrometer range.
To precisely describe fine patterns by exposing a substrate to an electron beam, the effect of the scattered electrons and the reflected electrons must be satisfactorily corrected. Therefore, heretofore, both the exposure dosage and the pattern size to be exposed has to be corrected when a predetermined pattern was to be described by the exposure of electron beams on a resist formed on a substrate, such as a semiconductor substrate or a mask substrate that was to be treated.
When an electron beam, accelerated at a voltage of about 20 to 25 kV, is projected onto a substrate, the electrons reflected by the substrate are scattered around an incidence point were incident electrons fall. It is known that the scattering radius of the reflected electrons is about 2.5 .mu.m when a voltage within the above-mentioned range is applied.
The scattering radius is nearly equal to the pattern dimensions and pattern gaps of modern semiconductor elements and, hence, imposes a serious problem from the standpoint of correcting the exposure. That is, the exposure dosage of the resist is given as the sum of incident electrons and the amount reflected electrons. Here, however, the amount of reflected electrons varies depending upon the size of the pattern and the density. Therefore, the effect of the reflected electrons is calculated for the individual patterns that are to be described in order to determine the amount of correction for both the exposure dosage and the pattern size to be exposed, thereby involving a series of cumbersome operations.